Fix according to comments

src/librustc/lint/builtin.rs

@@ -280,7 +280,7 @@ declare_lint! {
 }
 
 declare_lint! {
-    pub DUPLICATE_ASSOCIATED_TYPE_BINDING,
+    pub DUPLICATE_ASSOCIATED_TYPE_BINDINGS,
     Warn,
     "warns about duplicate associated type bindings in generics"
 }
@@ -336,7 +336,7 @@ impl LintPass for HardwiredLints {
             BARE_TRAIT_OBJECT,
             ABSOLUTE_PATH_NOT_STARTING_WITH_CRATE,
             UNSTABLE_NAME_COLLISION,
-            DUPLICATE_ASSOCIATED_TYPE_BINDING,
+            DUPLICATE_ASSOCIATED_TYPE_BINDINGS,
         )
     }
 }

src/librustc_lint/lib.rs

@@ -283,6 +283,11 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             reference: "issue TBD",
             edition: Some(Edition::Edition2018),
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(DUPLICATE_ASSOCIATED_TYPE_BINDINGS),
+            reference: "issue #50589 <https://github.com/rust-lang/rust/issues/50589>",
+            edition: None,
+        },
         ]);
 
     // Register renamed and removed lints

src/librustc_typeck/astconv.rs

@@ -406,16 +406,6 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                     trait_ref.ref_id, poly_trait_ref, binding, speculative, &mut dup_bindings);
             predicate.ok() // ok to ignore Err() because ErrorReported (see above)
         }));
-        for (_id, spans) in dup_bindings {
-            if spans.len() > 1 {
-                self.tcx().struct_span_lint_node(
-                        ::rustc::lint::builtin::DUPLICATE_ASSOCIATED_TYPE_BINDING,
-                        trait_ref.ref_id,
-                        spans,
-                        "duplicate associated type binding"
-                    ).emit();
-            }
-        }
 
         debug!("ast_path_to_poly_trait_ref({:?}, projections={:?}) -> {:?}",
                trait_ref, poly_projections, poly_trait_ref);
@@ -499,7 +489,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
         trait_ref: ty::PolyTraitRef<'tcx>,
         binding: &ConvertedBinding<'tcx>,
         speculative: bool,
-        dup_bindings: &mut FxHashMap<DefId, Vec<Span>>)
+        dup_bindings: &mut FxHashMap<DefId, Span>)
         -> Result<ty::PolyProjectionPredicate<'tcx>, ErrorReported>
     {
         let tcx = self.tcx();
@@ -577,7 +567,21 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
             tcx.sess.span_err(binding.span, &msg);
         }
         tcx.check_stability(assoc_ty.def_id, Some(ref_id), binding.span);
-        dup_bindings.entry(assoc_ty.def_id).or_insert(Vec::new()).push(binding.span);
+
+        dup_bindings.entry(assoc_ty.def_id)
+            .and_modify(|prev_span| {
+                let mut err = self.tcx().struct_span_lint_node(
+                    ::rustc::lint::builtin::DUPLICATE_ASSOCIATED_TYPE_BINDINGS,
+                    ref_id,
+                    binding.span,
+                    &format!("associated type binding `{}` specified more than once",
+                            binding.item_name)
+                );
+                err.span_label(binding.span, "used more than once");
+                err.span_label(*prev_span, format!("first use of `{}`", binding.item_name));
+                err.emit();
+            })
+            .or_insert(binding.span);
 
         Ok(candidate.map_bound(|trait_ref| {
             ty::ProjectionPredicate {

src/test/ui/lint/issue-50589-multiple-associated-types.stderr

@@ -1,14 +1,23 @@
-warning: duplicate associated type binding
+warning: associated type binding `Item` specified more than once
-  --> $DIR/issue-50589-multiple-associated-types.rs:17:28
+  --> $DIR/issue-50589-multiple-associated-types.rs:17:39
    |
 LL | fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
-   |                            ^^^^^^^^^  ^^^^^^^^^^^
+   |                            ---------  ^^^^^^^^^^^ used more than once
+   |                            |
+   |                            first use of `Item`
    |
-   = note: #[warn(duplicate_associated_type_binding)] on by default
+   = note: #[warn(duplicate_associated_type_bindings)] on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #50589 <https://github.com/rust-lang/rust/issues/50589>
 
-warning: duplicate associated type binding
+warning: associated type binding `Item` specified more than once
-  --> $DIR/issue-50589-multiple-associated-types.rs:17:28
+  --> $DIR/issue-50589-multiple-associated-types.rs:17:39
    |
 LL | fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
-   |                            ^^^^^^^^^  ^^^^^^^^^^^
+   |                            ---------  ^^^^^^^^^^^ used more than once
+   |                            |
+   |                            first use of `Item`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #50589 <https://github.com/rust-lang/rust/issues/50589>